Ceramic shell molds are prepared using patterns that are replicas of the parts to be cast and which may include the necessary gates and risers. The patterns are formed of any expendable materials, such as wax or a suitable synthetic resin or blends of wax and resin. These patterns are attached to a central support or sprue member to form what is known as a "set up" or "tree". An example of a "set up" is illustrated and described in U.S. Pat. No. 3,424,227, owned by the present assignee and hereby incorporated by reference.
The formation of a shell mold around the tree is generally accomplished by applying a refractory slurry coating of controlled viscosity followed by directional draining to coat the patterns completely. After draining excess slurry from the tree, the slurry coating is sanded or stuccoed while wet with coarser refractory materials. This layer is hardened, as by forced air drying at room temperature. The result is a layer of ceramic material having refractory particles embedded in the surface. After the first ceramic layer is sufficiently hard and dry, the steps of coating, draining, stuccoing and drying are repeated until a refractory shell having a sufficient thickness to resist the stresses occurring in subsequent operations has been built up around the tree. In a subsequent pattern removal operation, the tree, including the patterns, is removed from the shell mold and it is prepared for the casting operation.
In the past, the coating of set ups has been labor intensive. Attempts have been made to automate the coating process. In U.S. Pat. Nos. 3,278,998 and 4,295,444 machines for carrying set ups through a series of coating steps are illustrated. In general, both machines are rotary in nature and carry the set ups being coated in a circular path with the various processing stations located at predetermined intervals.